Known methods for the desiliconization and dephosphorization of molten hot metal for steel making which is flowing in a delivery trough of a blast furnace by adding thereto a powdered refining agent, may be roughly classified into the following 4 methods. The same classification may be made of known methods for the preliminary treatment of molten hot metal continuously caused to pass in a horizontally disposed elongated vessel from one end to another.
(1). As diagrammatically shown in FIG. 1, a top placing method wherein a powdered refining agent 2 is sprayed by its own weight on the surface of molten hot metal 1 continuously flowing in a trough.
(2). As shown in FIG. 2, a top blowing projection method wherein a powdered refining agent 2 is projected onto the surface of molten hot metal 1 continuously flowing in a trough by use of a non-oxidizing carrier gas 4 through a lance 3 disposed with its front end positioned above the surface of the molten hot metal.
(3). As shown in FIG. 3, a top blowing injection method wherein a powdered refining agent 2 is injected into molten hot metal 1 continuously flowing in a trough by use of a non-oxidizing carrier gas 4 through a lance 5 disposed with its front end positioned below the surface of the molten hot metal.
The top blowing projection method (2) above is disclosed in JP B 61-45,681.
Besides the above-mentioned methods, we have proposed in JP A 60-177,114, JP A 60-177,117 and JP A 60-181,212 wherein a powdered refining agent is injected into molten hot metal by use of an oxygen-rich gas, instead of a non-oxidizing gas, as a carrier, wherein the mixture of the oxygen-rich gas and the powdered refining agent is bottom blown through single-pipe nozzles instead of tuyeres of a double tube structure. In these methods solid products formed by reaction of the hot metal and refining agent accumulate on front ends of the single-pipe nozzles, and the injection can be efficiently carried out.